Dynamic loudspeaker driving apparatus

ABSTRACT

The dynamic loudspeaker driving apparatus consists of at least a power amplifier and a feedback circuit. The power amplifier amplifies an input signal so that the dynamic loudspeaker is driven by the amplified input signal. A detecting circuit is further provided in order to accurately detect a motional voltage produced at an equivalent motional impedance of dynamic loudspeaker, and the feedback circuit negatively feedbacks the detected motional voltage to the power amplifier so that distortions due to a transient response of a vibration system of dynamic loudspeaker will be eliminated. The amplified input signal is supplied to a first terminal of dynamic loudspeaker and a voltage at a second input terminal of dynamic loudspeaker is supplied to the feedback circuit, and impedance components other than the equivalent motional impedance can be canceled. In addition, it is possible to be further provided with a filter circuit having a frequency response characteristics which can be obtained by electrically simulating a voltage transmission characteristic against the equivalent motional impedance of dynamic loudspeaker. Thus, the input signal can be given with a desirable frequency characteristic by the filter circuit and then supplied to the power amplifier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a dynamic loudspeaker drivingapparatus, and more particularly to a dynamic loudspeaker drivingapparatus which can reduce levels of distortions in sound from a dynamicloudspeaker.

2. Prior Art

In general, a feedback circuit is arranged between input and output of apower amplifier provided within an amplifier unit of an audio device. Byuse of this feedback circuit, it is possible to reduce levels of noisesand distortion components included in an output signal of the poweramplifier.

In addition, the amplifier unit of the audio device may also be providedwith a motional feedback circuit (hereinafter, referred to as MFBcircuit) which feedbacks a signal corresponding to a vibration of adynamic loudspeaker so as to reduce a distortion in an operation of theloudspeaker. Theoretically, motional voltage must be applied to amotional impedance of the dynamic loudspeaker, and the MFB circuitnegatively feedbacks such motional voltage to the input of the poweramplifier.

The above-mentioned motional impedance can be represented by ZM of anelectrically equivalent circuit of the dynamic loudspeaker (hereinafter,referred simply to as a loudspeaker) shown in FIG. 1. In FIG. 1, Rvdesignates a dc resistance component of a voice coil, and Lv designatesan inductance component of the voice coil. In FIG. 2, a solid linedesignates voltage Vi supplied to the dynamic loudspeaker, while a shortdashes line designates motional voltage VM which is produced at theequivalent motional impedance ZM representative of a vibration system ofthe dynamic loudspeaker. The operating distortion of the vibrationsystem of the loudspeaker represents a transient response component ofthe motional voltage VM.

When the MFB circuit is provided to the dynamic loudspeaker, thenegative feedback quantity must become extremely large at thefrequencies in the vicinity of a lowest resonance frequency of thedynamic loudspeaker. Hence, it is avoided to provide too much negativefeedback quantity for the MFB circuit. In general, a frequencycharacteristic of the dynamic loudspeaker provided with the MFB circuithas a tendency that the frequency response characteristic must be easilylowered at low frequencies at which the negative feedback quantity mustbe concentrated. In order to prevent such frequency responsecharacteristic from being lowered at low frequencies, a compensatinglow-ass filter circuit (i.e., compensating LPF circuit) isconventionally provided at an input side of the dynamic loudspeaker sothat the frequency response characteristic will be raised at the lowfrequencies. However, it is impossible to obtain a perfect compensatingcharacteristic from such LPF circuit.

FIG. 3 shows an example of a conventional dynamic loudspeaker drivingapparatus providing the above-mentioned compensating LPF circuit. InFIG. 3, a feedback circuit 2 is connected between input and output sidesof a power amplifier 1. In this case, a feedback ratio b of the feedbackcircuit 2 is set further smaller than one, while a gain of the poweramplifier 1 is set further larger than one. Meanwhile, a dynamic speaker3 and three resistors 4 to 6 constitute a bridge circuit 7. An outputsignal Es of this bridge circuit 7 diagrammatically corresponds to themotional voltage of the dynamic speaker 3, and such signal Es isdetected by a transformer 8. A part of a detection signal outputted fromthe transformer 8 is feedbacked to the input side of power amplifier 1.In the circuit shown in FIG. 3, the resistors 4 to 6 and the transformer8 represent the MFB circuit.

In addition, a compensating LPF circuit 9 is provided at input side ofthe power amplifier 1, and lowering of low frequency characteristics ofthe circuit shown in FIG. 3 is improved and compensated by the MFBcircuit. More specifically, the compensating LPF circuit 9 adequatelyraises a signal level of input signal Vi in the low frequency range, andthe lowering of the low frequency characteristics is improved.

The MFB circuit used in the conventional audio amplifier unit isexclusively used for reducing distortions and noises included in asignal outputted from the power amplifier. However, such MFB circuit isnot used for perfectly eliminating distortions due to the transientresponse of the vibration system of the dynamic loudspeaker at all. Inshort, the main portion of the conventional dynamic loudspeaker drivingapparatus is the negative feedback portion, and the MFB circuit ismerely used as an auxiliary circuit of the dynamic loudspeaker drivingapparatus.

As shown in FIG. 3, the MFB circuit is a detection circuit constitutedby the transformer and the bridge circuit consisting of resistors only.Hence, detection voltage detected by this detection circuit is notidentical to the motional voltage in a strict sense. In other words, thedetection voltage and the motional voltage are different in waveform,peak value and phase. For this reason, it is naturally impossible toprovide much negative feedback, and the over-all frequencycharacteristics must be irregularly varied. Hence, the characteristicswhich must be given to the compensating LPF circuit must be extremelycomplicated, so that it is impossible to compensate the frequencycharacteristic of the dynamic loudspeaker with accuracy. Therefore, theconventional dynamic loudspeaker driving apparatus can only provide thecircuit which can adequately raise the output level in the low frequencyrange.

As described heretofore, in the conventional audio amplifier unit, it isimpossible to perfectly eliminate the all distortions due to thetransient response of the vibration system of the dynamic loudspeaker.

Meanwhile, the conventional MFB circuit can use a pressure sensor, atemperature sensor, a microphone or other sensors in order to detect themotional voltage. Instead of using the above-mentioned sensors, a bridgecircuit can be used for detecting the motional voltage produced at avoice coil of the loudspeaker, as described before. These techniques aredisclosed in a monthly magazine "Radio Technique" published in Japan;October Issue and November Issue in 1984, and February Issue in 1985,for example.

However, in the above-mentioned MFB circuit using the sensors, a phaserevolution of a detection output of such sensor must be increased, forexample. Hence, there must be a limit of a feedback quantity due to anability of the sensor. If the feedback quantity is set large, the MFBcircuit will oscillate by itself. As a result, the conventional MFBcircuit is disadvantageous in that a distortion reducing effect of theloudspeaker must become small.

On the other hand, the MFB circuit using the bridge circuit isdisadvantageous in that the circuit constitution thereof must becomplicated.

As described heretofore, the conventional dynamic loudspeaker drivingapparatus adopting the MFB circuit must detect the motional voltage. Forthis reason, it is impossible to sufficiently reduce the levels of thedistortions of the loudspeaker.

SUMMARY OF THE INVENTION

It is accordingly a primary object of the present invention to provide adynamic loudspeaker driving apparatus which can detect the motionalvoltage with accuracy and then negatively feedback the detected motionalvoltage by 100% so that the distortions due to the transient response ofthe vibration system of the dynamic loudspeaker will be perfectlyeliminated.

It is another object of the present invention to provide a dynamicloudspeaker driving apparatus which is constituted not to detect themotional voltage but to cancel impedance components other than anequivalent motional impedance of the vibration system of the dynamicloudspeaker so that it is possible to perfectly eliminate thedistortions due to the transient response of the vibration system of thedynamic loudspeaker.

In a first aspect of the invention, there is provided a dynamicloudspeaker driving apparatus comprising:

(a) an amplifier having a large open-loop-gain for driving a dynamicspeaker;

(b) detecting means for detecting a motional voltage applied to anequivalent motional impedance of the dynamic speaker;

(c) feedback means for negatively feedbacking the motional voltage to aninput terminal of said amplifier by a transmission gain "1"; and

(d) input means for supplying an input signal to the input terminal ofthe amplifier via a filter circuit which electrically simulates avoltage transmission characteristics against the equivalent motionalimpedance of dynamic speaker.

In a second aspect of the invention, there is provided a dynamicloudspeaker driving apparatus for amplifying an input signal and drivinga dynamic loudspeaker by the amplified input signal so that impedancecomponents other than an equivalent motional impedance of the dynamicloudspeaker can be canceled.

In a third aspect of the invention, there is provided a dynamicloudspeaker driving apparatus comprising:

(a) a filter circuit having a frequency response characteristic which isobtained by electrically simulating a voltage transmissioncharacteristic against an equivalent motional impedance of a dynamicloudspeaker, the filter circuit giving a desirable frequencycompensating characteristic to an input signal; and

(b) driving means having a negative output impedance which can cancelimpedance components other than the equivalent motional impedance, thedriving means driving the dynamic loudspeaker by an output signal of thefilter circuit.

In a fourth aspect of the invention, there is provided a dynamicloudspeaker driving apparatus comprising:

(a) a power amplifier for amplifying an input signal so that theamplified input signal is supplied to a first input terminal of adynamic loudspeaker having an equivalent motional impedance; and

(b) a servo amplifier for negatively feedbacking a voltage at a secondinput terminal of the dynamic loudspeaker to the power amplifier,whereby impedance components other than the equivalent motionalimpedance can be canceled.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will be apparentfrom the following description, reference being had to the accompanyingdrawings wherein preferred embodiments of the present invention areclearly shown.

In the drawings:

FIG. 1 is a circuit diagram showing an electrically equivalent circuitof the speaker;

FIG. 2 shows waveforms of the input voltage supplied to the loudspeakerand the motional voltage applied to the equivalent motional impedance ofthe vibration system of the loudspeaker;

FIG. 3 is a circuit diagram showing an electric constitution of theconventional dynamic loudspeaker driving apparatus;

FIG. 4 is a block diagram showing a basic constitution of a firstembodiment of the present invention;

FIGS. 5A and 5B and FIGS. 6A to 6C show frequency responsecharacteristics for explaining an operation of the first embodiment;

FIG. 7 is a circuit diagram showing an electric first embodiment;

FIGS. 8A and 8B are circuit diagrams for explaining functions of abridge detection circuit shown in FIG. 7;

FIG. 9 is a circuit diagram showing an essential constitution of thedynamic loudspeaker driving apparatus according to a second embodimentof the present invention;

FIG. 10 is a circuit diagram showing the second embodiment of thepresent invention;

FIG. 11 is a circuit diagram showing an embodiment of an essentialportion of the second embodiment;

FIG. 12 is a circuit diagram showing a concrete constitution of thesecond embodiment;

FIG. 13 is a circuit diagram showing a modified example of the secondembodiment;

FIGS. 14A to 14C are graphs showing frequency characteristics forexplaining an operation of the circuit shown in FIG. 13;

FIG. 15 is a circuit diagram showing a third embodiment of the presentinvention;

FIG. 16 is a graph showing frequency characteristics of the thirdembodiment; and

FIG. 17 is a circuit diagram showing a concrete constitution of thefilter circuit 110 of the third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, description will be given with respect to preferred embodiments ofthe present invention in conjunction with FIGS. 4 to 17, wherein likereference characters designate like or corresponding parts throughoutthe several drawings.

[A]FIRST EMBODIMENT

First, description will be given with respect to a basic constitution ofa first embodiment of the present invention. FIG. 4 is a block diagramshowing the basic constitution of the first embodiment of the presentinvention. In FIG. 4, the motional voltage VM is applied to theequivalent motional impedance ZM of the vibration system of the dynamicspeaker (or dynamic loudspeaker) 23, and such motional voltage VM isdirectly supplied to an inverting input terminal of power amplifier 21,whereby the motional voltage VM will be negatively feedbacked by 100%.Hence, a system AP consisting of the power amplifier 21 and the dynamicspeaker 23 can be considered as an equivalent voltage amplifier having avoltage gain "1" against the motional impedance ZM.

In addition, 20 designates a band-pass filter (BPF) circuit whichconstitutes input means of the first embodiment. The reasons why suchBPF circuit 20 is provided will be described as follows.

In general, when the constant voltage is applied to the input terminalof the dynamic loudspeaker to thereby drive the dynamic loudspeaker withthe constant voltage, it is possible to obtain a flat curve of tonepressure vs frequency characteristic as shown in FIG. 5A. In this case,a relation between the motional voltage VM and the frequency within thedynamic loudspeaker can be shown in FIG. 6A. In FIGS. 6A to 6C, each ofhatching parts represents actual acoustic energy.

On the other hand, when the motional voltage VM is negatively feedbackedby 100%, a curve of tone pressure vs frequency characteristic does notbecome flat and the tone pressure in the low frequency range must belowered as shown in FIG. 5B. In this case, the relation between themotional voltage VM and the frequency will be as shown in FIG. 6B,wherein the motional voltage VM directly corresponds to the input signalVi perfectly and thus the variation of motional voltage VM itself isperfectly suppressed. As a result, the distortions in an operation ofthe loudspeaker are prevented from being caused. However, in the casewhere the motional voltage VM is negatively feedbacked by 100% as shownin FIG. 6B, the curve of tone pressure vs frequency characteristic doesnot become flat as shown in FIG. 5B. For this reason, the waveform ofinput signal Vi is modified as shown in FIG. 6C by the BPF circuit 20 sothat the waveform of motional voltage VM will become equivalent to thewaveform in case shown in FIG. 5A. In other words, the BPF circuit 20provided to the input side of the power amplifier 21 is the circuitwhich can electrically simulate the voltage transmission characteristicsagainst the motional impedance of the vibration system of the dynamicspeaker 23. Due to this BPF circuit 20, the dynamic loudspeaker drivingapparatus provided with the MFB circuit can present the flat curve oftone pressure vs frequency characteristic as shown in FIG. 5A.

As described heretofore, the first embodiment has a circuit constitutionprovided with the system in which the motional voltage VM is negativelyfeedbacked by 100% between the power amplifier 21 and the dynamicspeaker 23. Due to this system, the first embodiment can perfectlyeliminate the distortions caused by the transient response of thevibration system of the dynamic speaker 23. In addition, the firstembodiment simulates the voltage transmission characteristics of theconventional dynamic loudspeaker at the input side of power amplifier21.

Next, description will be given with respect to the first embodiment indetail in conjunction with FIGS. 7, 8A and 8B. FIG. 7 is a circuitdiagram showing an electric constitution of the first embodiment.

In FIG. 7, a first fixed terminal 11a of variable resistor 11 isconnected to a signal input terminal 10 via a resistor 12, while asecond fixed terminal 11b thereof is connected to a first terminal ofresistor 13. In addition, a slider terminal 11c of variable resistor 11is connected to an input terminal of amplifier 14. In this case,resistance Ra denotes resistance combined by resistance of resistor 12and resistance between the terminals 11a and 11c of variable resistor11, while resistance Rb denotes resistance combined by resistance ofresistor 13 and resistance between the terminals 11b and 11c of variableresistor 11. The amplifier 14 is designed to have a voltage gain "+1".An output terminal of amplifier 14 is connected to a first terminal ofcapacitor 15 (having capacitance C0), while a second terminal ofcapacitor 15 is connected to a first terminal of resistor 16 (havingresistance R0). A second terminal of resistor 16 is grounded via aparallel circuit consisting of a resistor 17 (having resistance R0) anda capacitor 18 (having capacitance C0) and then connected to an inputterminal of amplifier 19. This amplifier 19 is designed to have avoltage gain "+3". In addition, an output terminal of amplifier 19 isconnected to a second terminal of resistor 13 and then connected toanon-inverting input terminal of amplifier 21a. The BPF circuit 20 isconstituted by the amplifiers 14 and 19, the variable resistor 11, theresistors 12, 13, 16 and 17, the capacitors 15 and 18 as describedabove.

Next, description will be given with respect to characteristics of BPFcircuit 20. This BPF circuit has a resonance frequency f1 which isdetermined by the resistances of resistors 16 and 17, the capacitancesof capacitors 15 and 18. In short, the resonance frequency f1 isrepresented by the following formula (1).

    f1=1/2πC0·R0                                   (1)

In addition, a sharpness Q of resonance is represented by the followingformula (2).

    Q=(1+Ra/Rb)/3                                              (2)

By suitably selecting the capacitances of capacitors 15 and 18, theresistances of resistors 16 and 17 in the BPF circuit 20, the resonancefrequency f1 of the BPF circuit 20 can be coincided with the lowestresonance frequency f0 of the dynamic speaker 23. By adjusting thevariable resistor 11, a frequency bandwidth in resonance characteristicscan be arbitrarily varied. In other words, in the case where theresistance Ra is set larger than the resistance Rb by adjusting thevariable resistor 11, the value Q becomes large so that a frequencybandwidth of resonance characteristics will become narrow. On thecontrary, in the case where the resistance Ra is set smaller than theresistance Rb, the value Q becomes small so that the frequency bandwidthof resonance characteristics will become wide. Accordingly, by using theBPF circuit 20, the resonance characteristics of input signal Vi can besimulated to the voltage transmission characteristics against themotional impedance of dynamic speaker 23 with accuracy.

In FIG. 7, the power amplifier 21 is constituted by the voltageamplifier 21a having a large open-loop-gain and a power stage consistingof a NPN type transistor 21b and a PNP type transistor 21c. An outputterminal of amplifier 21a is connected to both base terminals oftransistors 21b and 21c. Both emitter terminals of transistors 21b and21c are connected in common to constitute an output terminal of poweramplifier 21.

The output terminal of power amplifier 21 is connected to a firstterminal of dynamic speaker 23, and this output terminal is grounded viaa resistor 24 (having resistance of a.Rv; "a" denotes an arbitrarycoefficient), a resistor 25 (having resistance of a.Rs/2) and a resistor26 (having resistance of a.Rs/2) in series. In this case, a capacitor 27(having capacitance Cv1=Lv/(a.Rs.Rv)) is connected in parallel to aserial circuit consisting of the resistors 25 and 26. In addition, asecond terminal of dynamic speaker 23 is grounded via a resistor 31(having resistance Rs). The dynamic speaker 23 can be electricallyrepresented by an equivalent circuit which is constituted by a serialcircuit consisting of a voice coil resistor 28 (having resistance Rv), avoice coil inductance 29 (having inductance Lv) and an equivalentcircuit 30 of a mechanical vibration system of dynamic speaker 23. Thisequivalent circuit 30, i.e., the motional impedance, can be representedby a parallel circuit consisting of a resistor 30a, a capacitor 30b anda coil (inductance) 30c.

The above-mentioned dynamic speaker 23, the resistors 24, 25, 26 and 31,the capacitor 27 constitute a bridge circuit 32.

Next, description will be given with respect to functions of the bridgecircuit 32. The combined resistance of the resistors 24 to 26 within thebridge circuit 32 can be represented by (a.Rv+a.Rs/2+a.Rs/2). Suchcombined resistance is set sufficiently larger than another combinedresistance (Rv+Rs) of the resistors 28 and 31, and the resistance Rs ofresistor 31 is set sufficiently smaller than the resistance Rv ofresistor 28. Meanwhile, a condition as described by the followingformula (3) is set between the dynamic speaker 23 and the resistors 24,25, 26 and 31.

    (a·Rv)/(a·Rs)=Rv/Rs                      (3)

By adequately setting the resistances of resistors as described above,it becomes possible to accurately detect the motional voltage VM betweena connection point P4 formed between the resistors 25 and 26 and anotherconnection point P2 formed between the resistor 31 and the secondterminal of dynamic speaker 23, which will be described later.

Next, the above-mentioned connection point P4 between the resistors 25and 26 is connected to a non-inverting input terminal of amplifier 34.In addition, the connection point P2 between the dynamic speaker 23 andthe resistor 31 is connected to an inverting input terminal of amplifier34 via a resistor 35 (having resistance r), and this connection point P2is also connected to a first terminal of resistor 36 (having resistancer). A second terminal of resistor 36 is connected to an output terminalof amplifier 37. This amplifier 37 is designed to have a voltage gain"+1". An input terminal of amplifier 37 is connected to an outputterminal of amplifier 34 via a resistor 38 (having resistance b.Rv; "b"denotes an arbitrary coefficient), and this input terminal of amplifier37 is grounded via a parallel circuit consisting of a resistor 39(having resistance b.Rs) and a capacitor 40 (having capacitanceCv2=Lv/(b.Rs.Rv)). The bridge circuit 32, the amplifiers 34 and 37, theresistors 35, 36, 38 and 39, and the capacitor 40 constitute a bridgeamplifier 41. This bridge amplifier 41 corresponds to detecting means.

The output terminal of amplifier 34 is connected to a first terminal ofcapacitor 42 (having capacitance Cf). A second terminal of capacitor 42is connected to a first terminal of resistor 43 (having resistance Rf)and also connected to the inverting input terminal of amplifier 21awithin the power amplifier 21. A second terminal of resistor 43 isconnected to the output terminal of power amplifier 21. The capacitor 42is used for blocking a direct current, and the resistor 43 is used as afeedback resistor.

Next, description will be given with respect to a detecting principle ofthe motional voltage VM by use of the bridge amplifier 41.

First, in the bridge circuit 32 shown in FIG. 8A, the relation betweenthese voltages V0 to V4 can be represented by the following formula (4).In this formula, V0 denotes a voltage supplied from the power amplifier21, V1 denotes a voltage supplied to the non-inverting input terminal ofamplifier 34, V2 denotes a voltage at the connection point P2, V3denotes a voltage at the output terminal of amplifier 37 and V4 denotesa voltage at the output terminal of amplifier 34.

    V3=V4·(b·Rs//Cv2)/(b·Rs//Cv2+b·Rv) =V4·Rs/(Rs+Rv+jwLv)                              (4)

wherein

Cv2=Lv/(b.Rs.Rv) and "Rs//Cv2" means a combined impedance of parallelcircuit consisting of resistance Rs and capacitance Cv.

In addition, the following formula (5) can be obtained based on acharacteristic of operational amplifier with feedback.

    V1 =(r·V2+r·V3)/(r+r)=(V2+V3)/2∴V3=2·V1-V2(5)

Next, the voltages V1 and V2 can be obtained by referring to FIG. 8B asdescribed by the following formulae (6) and (7).

    2·V1=V0 (a·Rs//Cv1)/(a·Rs//Cv1+a·Rv) =V0·Rs//(Rs+Rv+jwLv)                             (6)

wherein

Cv1=Lv/(a·Rs·Rv).

    V2=(V0-VM)·Rs/(Rs+Rv+jwLv)                        (7)

When the above-mentioned formulae (6) and (7) are put in the formula(5), the following formula (8) can be obtained.

    V3=VM·Rs/(Rs+Rv+jwLv)                             (8)

Thus, the following formula (9) can be obtained from the formulae (4)and (8).

    V4=VM                                                      (9)

Accordingly, the motional voltage VM of the dynamic speaker 23 can beobtained from the output of amplifier 34 with accuracy.

Next, description will be given with respect to the operation of thefirst embodiment in conjunction with FIG. 7.

First, the input signal Vi applied to the signal input terminal 10 issupplied to the BPF circuit 20 wherein the signal level of input signalVi is raised in the resonance frequency f1. More specifically, a signal(Vi+VM) outputted from the BPF circuit 20 has a frequency bandwidthcharacteristics which are obtained by simulating the voltagetransmission characteristics of the dynamic speaker 23. This signal(Vi+VM) is supplied to the non-inverting input terminal of amplifier 21awithin the power amplifier 21 wherein the signal (Vi+VM) is amplified.Then, the amplified signal is supplied to the dynamic speaker 23,whereby the dynamic speaker 23 will be driven. At this time, themotional voltage VM is produced between the both terminals of equivalentcircuit 30 of the dynamic speaker 23. Such motional voltage VM isdetected by the bridge amplifier 41, and the detected motional voltageVM is supplied to the inverting input terminal of amplifier 21a via thecapacitor 42. In short, the motional voltage VM is feedbacked by 100%.

Since the motional voltage VM is feedbacked by 100% as described above,it is possible to perfectly eliminate the distortions due to thetransient response of the vibration system of dynamic speaker 23. Inaddition, the first embodiment simulates the voltage transmissioncharacteristics of dynamic speaker 23 at the input stage thereof. Hence,similar to the conventional apparatus, the first embodiment can realizethe flat curve of tone pressure vs frequency characteristic. Moreover,the frequency range of the frequency characteristic of the firstembodiment can be stretched to further lower frequency range by varyingthe voltage transmission characteristics at the input stage, regardlessof the lowest resonance frequency of the frequency characteristic.

[B]SECOND EMBODIMENT

FIG. 9 is a circuit diagram showing an essential constitution of thedynamic loudspeaker driving apparatus according to a second embodimentof the present invention.

In FIG. 9, an input terminal 101 applied with an input voltage Vi isconnected to an inverting input terminal of an operational amplifier (ora power amplifier) 102 via a resister R1. A non-inverting input terminalof the operational amplifier 102 is grounded, while the output terminalthereof is connected to a connection point between the resistor R1 andthe non-inverting input terminal thereof via a resistor R3. In addition,the output terminal of the operational amplifier 102 is grounded via aload 103 (which is a speaker, for example) having an impedance ZL and aresistor Rt in series. A connection point between the load 103 and theresistor Rt is connected to a connection point among the inverting inputterminal of the operational amplifier 102, the resistors R1 and R3 viaan amplifier (or a servo amplifier) 104 having gain "-A" and theresistor R2 in series.

In the above-mentioned constitution, when voltage Vo is applied betweenboth terminals of the load 103, a transmission characteristicrepresented by "-Vo/Vi" can be obtained from the following formula (10).

    -Vo/Vi=(R3/R1)·[1/]{1+(Rt/ZL)·(1-A·R3/R2)}](10)

Hence, an output impedance (or a drive impedance) Zo can be obtainedfrom the following formula (11).

    Zo=Rt(1-A·R3/R2)                                  (11)

According to the above formula (11), it is possible to set the value ofthe output impedance Zo to a negative value under a condition where avalue of A·R3/R2 is larger than one.

Next, description will be given with respect to a second embodiment ofthe present invention in conjunction with FIG. 10. This secondembodiment represents a case where the essential circuit shown in FIG. 9is applied to an actual speaker driving circuit. In FIG. 10, partsidentical to those shown in FIG. 9 will be designated by the samenumerals.

As shown in FIG. 10, a resistor R2a (having a resistance equal to thatof the resistor R2) is used instead of the resistor R3. As the amplifier104, a servo amplifier consisting of an operational amplifier 105,impedance loads 106 and 107 is used. Further, a dynamic speaker 108 isused instead of the load 103.

In FIG. 10, a connection point between the output terminal of theoperational amplifier 102 and the resistor R2a is connected to aterminal 108a of the dynamic speaker 108, while another terminal 108b ofthe dynamic speaker 108 is grounded via the resistor Rt. In addition,the terminal 108b is connected to an inverting input terminal of theoperational amplifier 105 via the impedance load 106 (having animpedance Z1), and a non-inverting input terminal of the operationalamplifier 105 is grounded. The output terminal of the operationalamplifier 105 is connected to a connection point between the invertinginput terminal thereof and the impedance load 106 via the impedance load107 (having an impedance Z2) and also connected to the resistor R2.

Meanwhile, in the speaker 108, Rv and Lv respectively designate a dcresistance and an inductance of a voice coil, and a resistor RM, acapacitor CM and a coil LM within a parallel circuit designaterespective components of a motional impedance ZM of a drive system ofthe speaker 108.

When relations of R2=R3 and A=Z2/Z1 are respectively put into theformulae (10) and (11), the transmission characteristic (-Vo/Vi) and theoutput impedance of the second embodiment can be obtained from thefollowing formulae (12) and (13).

    -Vo/Vi=(R2/R1)·[1/]{1+(Rt/ZL)·(1-Z2/Z1)}](12)

    Zo=Rt(1-Z2/Z1)                                             (13)

Next, description will be given with respect to a detailed constitutionof the servo amplifier 104 in conjunction with FIG. 11.

In order to drive the motional impedance ZM under a constant voltage,the value of the drive impedance Zo is to be set equal to a value of-(Rv+jwLv). When such relation is put into the formula (13), thefollowing relation can be obtained.

    -(Rv+jwLv)=Rt·(1-Z2/Z1) ∴Z2/Z1=(Rt+Rv)/Rt+jwLv/Rt(14)

Hence, a capacitance of the capacitor C1 and resistances of resistors R4and R5 can be set as follows.

R4 =k1·Rt

R5 =k1·(Rt+Rv)

C1 =C/k1

where C =Lv/[Rt·(Rt+Rv)] and k1 is set further larger than one.

When the circuits shown in FIGS. 10 and 11 are combined together, acircuit shown in FIG. 12 can be obtained. In this case, when thecondition represented by the formula (14) and a relation ofZL=Rv+jwLv+ZM are put into the formula (12), the following transmissioncharacteristic (-Vo/Vi) of formula (15) can be obtained.

    -Vo/Vi=R2/R1·[(Rv+jwLv+ZM)/Z]                     (15)

In addition, when a relation of VM/Vo=ZM/(Rv+jwLv+ZM) is put into thisformula (15), the transmission characteristic including the motionalimpedance ZM can be obtained from the following formula (16).

    -VM/Vi=R2/R1                                               (16)

Further, an output impedance Rd and a drive impedance Zd of the motionalimpedance ZM can be obtained as follows.

    Rd=-(Rv+jwLv)                                              (17)

    Zd=0                                                       (18)

Incidentally, as the setting method of circuit constants for setting thedrive impedance Zd equal to -(Rv+jwLv) in order to drive the motionalimpedance ZM under the constant voltage, modified methods other than themethod described before can be adopted. For example, impedance loads Z3and Z4 (not shown) can be used instead of the resistors R2 and R3 in thecircuit shown in FIG. 9, and constants of these impedance loads Z3 andZ4 can be set so that the value of the formula (11) will be setequivalent to the drive impedance Zd.

As known well, each of the value Q and a lowest resonance frequency f0has a value due to a resonance characteristic curve of the motionalimpedance ZM. However, when the speaker 108 is actually driven, there isa problem in that the above resonance characteristic curve (i.e., avariation of the motional impedance ZM) must be effective due to theresistance Rv of the voice coil and the output impedance Rd of theamplifier on the voltage transmission characteristics.

In order to solve such problem, the resonance impedance only must besubjected to a voltage drive by an amplifier having no output impedanceand infinite power, for example. In this case, the voltage between bothterminals of the resonance impedance is not effected by the value Q andthe silent resonance frequency f0 but identical to the input voltage. Inshort, it is not necessary to consider the value Q and the resonancefrequency f0 in this case. In addition, all movements of a vibrationplate of the actual loudspeaker is translated into an electromotiveforce between both terminals of the motional impedance ZM. Hence, bydriving the motional impedance ZM under the constant voltage, all freemovements of the vibration plate of the loudspeaker can be controlled.For this reason, the transient response of the vibration system can notbe caused at all, hence, it is possible to eliminate the distortions dueto such transient response.

As shown by FIG. 12 and formulae (16) to (18), the present invention candrive the motional impedance ZM by zero-ohm (or under the constantvoltage). However, the motional impedance ZM becomes extremely low atthe resonance frequency (f0). Hence, current supplying ability atdriving side is required to be large at the frequencies other than theresonance frequency f0.

By the way, it is possible to obtain an equivalent circuit as shown inFIG. 13 by simplifying the circuit shown in FIG. 12.

In FIG. 13, the input terminal 101 is connected to a connection point Pbetween the motional impedance ZM and the voice coil inductance Lv ofthe speaker 108 via the resistors R1 and R2 in series, while theterminal 108b is grounded. Meanwhile, an amplifier 109 having a negativeoutput impedance -(Rv+jwLv) is newly provided. The non-inverting inputterminal of this amplifier 109 is grounded, while the inverting inputterminal thereof is connected to a connection point between theresistors R1 and R2.

In general, the whole system of the dynamic speaker 108 including thevoice coil resistance Rv and inductance Lv has a tone pressure vsfrequency characteristic the curve of which is set to be flat under theconstant voltage. However, it is necessary to consider the potentials atthe input terminals 108a and 108b and the connection point P of thespeaker 108 shown in FIG. 13 in the actual case. In this case, themotional impedance ZM having the frequency characteristic shown by FIG.14A becomes extremely small at the frequencies other than the lowestresonance frequency f0. Hence, in order to set the voltage between theboth terminals of the motional impedance ZM to the constant voltage, adrive current I of the speaker 108 must be decreased in the vicinity ofthe resonance frequency f0 as shown in FIG. 14B. This drive current I isactually supplied to the speaker 108 via the voice coil resistance Rv,hence, a voltage V must be produced at the terminal 108a. This voltage Vbecomes extremely large at the frequencies other than the silentresonance frequency f0 as shown in FIG. 14C. For this reason, theamplifier 109 must be saturated soon.

The above-mentioned problem can be solved by using a filter circuit asdescribed in FIG. 7. More specifically, this filter circuit has a(frequency response) characteristic which can be obtained byelectrically simulating how the loudspeaker input voltage is transmittedin response to the motional impedance. In this case, the input signalvoltage Vi is supplied to the speaker 108 via this filter circuit.

[C]THIRD EMBODIMENT

FIG. 15 shows a diagrammatic circuit diagram of the third embodiment ofthe present invention which is further provided with the above-mentionedfilter circuit. In FIG. 15, 110 designates a filter circuit having afrequency response characteristic which can be obtained by electricallysimulating a voltage transmission characteristic of the speaker 108.More specifically, such filter circuit 110 includes a resistance k2·Rv,an inductance k2·Lv and a motional impedance k2·ZM (where k2 denotes anarbitrary constant value).

Due to this filter circuit 110, the voltage applied to the motionalimpedance ZM within the speaker 108 can have the frequencycharacteristic identical to that of the input voltage Vi in the casewhere the speaker 108 is driven by the input voltage Vi. For thisreason, it can be naturally said that the tone pressure vs frequencycharacteristic of the speaker 108 must have the flat curve. In addition,the input voltage of the amplifier 109 must be extremely low at thefrequencies except for the frequencies in the vicinity of the resonancefrequency f0 of the motional impedance ZM. Further, as described before,even if the circuit gain of the amplifier 109 becomes large at thefrequencies other than the resonance frequency f0, the output voltage ofthe amplifier 109 can not become so large.

Next, description will be given with respect to a concrete embodiment ofthe filter circuit 110 in conjunction with FIGS. 16 and 17. This filtercircuit 110 must have a frequency response characteristic F which issimilar to that of the speaker 108 as shown by a short dashes line inFIG. 16. In order to realize such frequency response characteristic F,this characteristic F is divided into a band-pass characteristic G1 andhigh-pass characteristics G2 to G4. By electrically simulating thesedivided characteristics, the circuit as shown in FIG. 17 can beconstituted. In FIG. 16, f1 to f3 designate respective cutofffrequencies of the above-mentioned high-pass characteristics G2 to G4.

In FIG. 17, 111 and 112 respectively designate input and output buffers;an amplifier 113, a resistor R (having a resistance of 470 kilo-ohm) anda capacitor C (having a capacitance of 0.0056 micro-farad) etc.constitute a band-pass filter having the band-pass characteristic G1;resistors r0 (having a resistance of 10 kilo-ohm), r1 (having aresistance of 22 kilo-ohm) and r2 (having a resistance of 68 kilo-ohm)and capacitors Ca (having a capacitance of 0.016 micro-farad), Cb(having a capacitance of 0.01 micro-farad) and Cc (having a capacitanceof 0.08 micro-farad) etc. constitute a circuit realizing the high-passcharacteristics G2 to G4. As other circuit elements, resistors Ry(having a resistance of 6.8 kilo-ohm), Rx (having a resistance of 1kilo-ohm), r3 (having a resistance of 1 kilo-ohm) and 2·r3 (having aresistance of 2 kilo-ohm) are provided.

In addition, the band-pass characteristic G1 has a time constant T1=R C;the high-pass characteristic G2 has a time constant T2=(r0+r1+r2)·Ca;the high-pass characteristic G3 has a time constant T3=(r0+r1)·Cb; andthe high-pass characteristic G4 has a time constant T4=Ca.r0. Further,as shown in FIG. 16, the high-pass characteristics G2 to G4 haverespective responses of r0/(r0+r1+r2), r0/(r0+r1) and r0/r0=1.

As described heretofore, the present invention is constituted so thatthe impedance components other than the equivalent motional impedance ofthe dynamic loudspeaker can be canceled. Hence, it becomes unnecessaryto consider the value Q and the lowest resonance frequency f0. Inaddition, it becomes possible to eliminate a low-frequency tonereproducing limitation due to the resonance frequency f0.

On the other hand, the present invention is provided with the filtercircuit and driving means. Such filter circuit has the frequencyresponse characteristic which can be obtained by electrically simulatingthe voltage transmission characteristic against the equivalent motionalimpedance of the dynamic loudspeaker, so that this filter circuit cangive a desirable frequency compensating characteristic to the inputsignal. The driving means has a negative output impedance which cancelsthe impedance components other than the equivalent motional impedance.This driving means drives the dynamic loudspeker by the input signalwhich is supplied thereto via the above filter circuit. Hence, it ispossible to arbitrarily raise the level of the low-frequencycharacteristic in principle by setting the level of the low-frequencycharacteristic large when setting the characteristics of the filtercircuit. Accordingly, it is possible to reproduce ultra-low-frequencytones by use of a small-size speaker.

Above is description of preferred embodiments of the present invention.This invention may be practiced or embodied in still other ways withoutdeparting from the spirit or essential character thereof. Therefore, thepreferred embodiments described herein are illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims and all variations which come within the meaning of the claimsare intended to be embraced therein.

What is claimed is:
 1. A dynamic loudspeaker driving apparatus comprising:(a) an amplifier having an open-loop-gain for driving a dynamic loudspeaker; (b) detecting means for detecting a motional voltage applied to an equivalent motional impedance of said dynamic loudspeaker; (c) feedback means for negatively feedbacking said motional voltage to an input terminal of said amplifier by a transmission gain "1"; and (d) input means for supplying an input signal to said input terminal of amplifier via a filter circuit which electrically simulates a voltage transmission characteristics against said equivalent motional impedance of dynamic loudspeaker.
 2. A dynamic loudspeaker driving apparatus according to claim 1 wherein said detecting means is a bridge circuit comprised of four impedance portion, one of which is an impedance of said dynamic loudspeaker including said equivalent motional impedance.
 3. A dynamic loudspeaker driving apparatus comprising:(a) a filter circuit having a frequency response characteristic which is obtained by electrically simulating a voltage transmission characteristic against an equivalent motional impedance of a dynamic loudspeaker, said filter circuit giving a desirable frequency compensating characteristic to an input signal; and (b) driving means having a negative output impedance which can cancel electrical impedance components of said filter circuit other than said equivalent motional impedance, said driving means driving said dynamic loadspeaker by an output signal of said filter circuit; wherein said filter circuit comprising at least a band-pass filter and a high-pass filter, said band-pass filter having a band-pass frequency range the center frequency of which is nearly set equal to a lowest resonance frequency of said dynamic loudspeaker, said high-pass filter having a cut-off frequency which is set higher than said lowest resonance frequency.
 4. A dynamic loudspeaker driving apparatus according to claim 3 wherein said driving means comprises(a) a power amplifier for amplifying said output signal of said filter circuit so that the amplified output signal is supplied to a first input terminal of said dynamic loadspeaker; and (b) a servo amplifier for negatively feedbacking a voltage at a second input terminal of said dynamic loudspeaker to said power amplifier.
 5. A dynamic loudspeaker driving apparatus according to claim 3 wherein said impedance components other than said equivalent motional impedance are identical to impedance components of a voice coil of said dynamic loudspeaker.
 6. A dynamic loudspeaker driving apparatus comprising:(a) a power amplifier comprising a first operational amplifier for amplifying an input signal so that the amplified input signal is supplied to a first input terminal of a dynamic loudspeaker having an equivalent motional impedance; and (b) a servo amplifier comprising an amplifier having a negative gain for negatively feedbacking a voltage at a second input terminal of said dynamic loudspeaker to said power amplifier whereby electrical impedance components of said power amplifier other than said equivalent motional impedance can be cancelled; said input signal being supplied to an inverting input terminal of said first operational amplifier via a first resistor, a non-inverting input terminal of said first operational amplifier being grounded, an output terminal of said servo amplifier being connected to said inverting input terminal of said first operational amplifier via a second resistor, an output terminal of said first operational amplifier being connected to said first input terminal of said dynamic loudspeaker and also connected to said inverting terminal input thereof via third resistor and an input terminal of said servo amplifier being connected to said second input terminal of said dynamic loudspeaker.
 7. A dynamic loudspeaker driving apparatus according to claim 6 wherein said servo amplifier comprises a second operational amplifier, first and second impedance loads,a non-inverting input terminal of said second operational amplifier being grounded, said second input terminal of said dynamic loudspeaker being connected to an inverting input terminal of said second operational amplifier via said first impedance load, an output terminal of said second operational amplifier being connected to said inverting input terminal of said first operational amplifier via said second resistor, said inverting input terminal of said second operational amplifier being connected to the output terminal thereof via said second impedance load. 